In an audio conferencing system, whether full-duplex or half-duplex, it is useful to keep track of the noise level in both the incoming (line-in) and the outgoing direction (line-out). For reasons related to echo cancellation though, speech activity in the opposite direction of the signal of interest (that is, near-end speech for line-in signal and far-end speech for line-out signal) may cause artificial fluctuations in the noise level that needs to be estimated. In other words, the absence of speech activity in the signal of interest does not guarantee that this portion of the signal represents the actual background noise of the signal of interest. Thus, where the signal of interest is the line-in signal, the echo canceller on the far-end side either shuts down its transmit signal (in the case of a half-duplex device), or applies a “Non Linear Processor” (in the case of a full-duplex device) during speech activity in the received signal (near-end speech). This results in signal level variations in the ‘line-in’ signal during such near end speech activity which is misinterpreted as far end noise due to the absence of far-end speech. A similar analysis applies to the noise level estimation of the line-out signal during far-end speech activity. In both cases, as indicated above, undesirable signal level variations result that may affect noise level estimations of the signal during speech (or tone) activity on the signal in the opposite direction.
Methods are well known in the art for tracking the level of the portions of a signal that are free of speech (or in-band tones) to perform noise level estimation. Thus, the prior art teaches the use of voice activity detection on a signal of interest to control noise level estimation on the signal. Example of such prior an systems are set forth in:    [1]“Noise signal prediction system”. Joji Kane and Akira Nohara . U.S. Pat. No. 5,295,225.    [2]“Noise suppression of acoustic signal in telephone set”. Toshio Yoshida and Michitaka Sisido. U.S. Pat. No. 5,617,472.    [3]“Method of detecting silence in a packetized voice stream”. Franck Beaucoup. Canadian Patent Application No 2,309,524, published Nov. 28, 2000.
None of the prior art, however, addresses the issue of noise level fluctuations due to speech activity on the signal in an opposite direction to the signal of interest. Consequently, the prior art systems discussed above may suffer from the aforementioned noise level fluctuations. The gravity of such consequences depends on the particular system; and in particular on how much tracking ability the application requires from the noise level estimation.